Crop domestication creates a refuge from parasitism for a native moth.
Abstract
Insect herbivores are generally more abundant in agroecosystems than in natural ecosystems. One possible cause is that artificial selection of crop plants may have disrupted parasitoid-herbivore interactions. We studied a native tritrophic system to determine if crop domestication has created a refuge for the sunflower moth Homoeosoma electellum from its major parasitoid Dolichogenidea homoeosomae. Parasitoid foraging behaviour on infested wild and agricultural sunflowers was studied in the greenhouse using event-recording software. Detailed observations were used on larval feeding location and parasitoid probing behaviour to determine if the lower parasitism on agricultural sunflowers was the result of a structural refuge or a change in parasitoid foraging behaviour. Greater than 20 times more larvae fed on seeds of agricultural sunflowers than on seeds of wild sunflowers. Seed-feeding enabled the larvae to escape parasitoids because their ovipositors were too weak to penetrate the seed. As a consequence, larvae were more likely to be parasitized on wild sunflowers than on agricultural sunflowers, and floret-feeding larvae were more likely to be parasitized than seed-feeding larvae. Parasitoids were more stimulated to probe for larvae on wild sunflowers than on agricultural sunflowers. However, they responded to larval feeding location rather than plant genotype. Thus, lower parasitism on agricultural sunflowers was due to a structural refuge for larvae rather than a change in parasitoid foraging behaviour. Crop domestication has created a structural refuge from parasitism. The presence of refuges on agricultural plants helps explain why some insects are difficult to regulate using biological control. Studying parasitoid activity on wild relatives of agricultural plants can identify how plant variation contributes to the creation of a refuge. Beneficial plant traits can then potentially be bred back into the crop germplasm in order to enhance the biological control of insect pests in agroecosystems.